There are three major types of brushless electric machines available for the electric vehicle (EV) and hybrid electric vehicle (HEV) drive systems. These are the induction machine, the PM machine, and the switched-reluctance machine.
Permanent magnet (PM) machines with and without reluctance paths have been recognized for having a high power density characteristic. A PM rotor does not generate copper losses. One drawback of the PM motor for the above-mentioned application is that the air gap flux produced by the PM rotor is limited, and therefore, a sophisticated approach is required for high speed, field weakening operation. Another constraint is that inductance is low, which means that current ripple must be controlled.
It is understood by those skilled in the art that a PM electric machine has the property of high efficiency and high power density. However, the air gap flux density of a PM machine is limited by the PM material, which is normally about 0.8 Teslas and below. A PM machine cannot operate at an air gap flux density as high as that of a switched reluctance machine. When the PM motor needs a weaker field with a reasonably good current waveform for high-speed operation, a sophisticated power electronics inverter is required.
When considering a radial gap configuration for undiffused, high strength operation, several problems have to be overcome. It is desirable to provide a compact design with a shape similar to a conventional radial gap machine and to include laminated rotor-core structure.
It would also be beneficial to further enhance the control of the field above that which is available with known PM rotor constructions. This would increase the motor torque. It is also an objective to accomplish this while retaining the compactness of the machine.
The enhanced field weakening can reduce the field strength at high speed to lower the back EMF produced in the winding. Therefore, for a specified DC link voltage, the speed range of the machine can be increased over what it otherwise would be. This will meet the compactness objective and allow simplification of the drive system requirements.
The permanent magnet (PM) motor is known to have higher power density among motors. However, the air-gap flux density of a PM motor is fixed due to the “permanent” nature of the magnet. The HSUB motor with the field enhancement and weakening capabilities can overcome the drawbacks of the PM motors.